Messi was good, but not god mode. His form does seem to take a dip for Argentina. Not as big of one as people say, but a definite dip. I have some thoughts on why that is. I think the players for Argentina don't go on as many forward runs as Barcelona players do. And they don't practice together nearly as much, so Messi is not quite so used to where they want the ball or when they will make runs. His passing game is really all about splitting open the defense by passing to a guy making a well-timed run. If players don't go on those runs that often and you are not sure exactly when they will do it, you'll end up making a lot less of those great passes. Of course, no one has much practice with their NT teammates, so logically this would affect everyone, not just Messi. But other greats players' passing games aren't reliant quite so much on perfect timing, so I think this hurts Messi more than it might hurt other players at the NT level. If his passing isn't going to make quite so much of a difference, Messi is left to try to make a difference through dribbling. But to dribble, you need space. Yes, even Messi needs space to dribble. Barcelona gets him space in two key ways. First, players make runs. This drags defenders away from Messi OR gives Messi an open teammate to pass to. Again, Argentina's players don't make those runs nearly so often, so the defenders can bunch up and take away Messi's space. Second, when defenders do bunch up like that, Messi typically tries to play one-two's with Barcelona teammates. You can get through a crowded area much more easily without the ball, so he 1-2's his way through crowds. But that takes really precise one-touch passing, the type of passing Barcelona players thrive on and practice every day. But his Argentina teammates just don't have the same skill set to do that. And without that, it becomes much harder to turn a dribbling move into a scoring or assisting opportunity. So Messi is stuck dribbling into crowds and having few defense-splitting passes open.
Basically, I think perhaps Messi's game is a little reliant on his teammates making well timed runs and having the touch to make 1-2's in crowded areas. And he doesn't get that as much for Argentina, so it's harder for him to make stuff happen. He is left dribbling into crowds, trying to make shots outside the box (i.e. outside of the crowds of defenders), and/or trying to get goals through those well-timed late run into the box he does. He can still be quite effective doing that (and I really hope that he will be in the rest of this Copa America), but it's a significant difference from what he can do at Barcelona.